| ||For the modified FEV strain in Fallout 2, see FEV Curling-13.|
Modified FEV is a modified strain of the Forced Evolutionary Virus developed by John Henry Eden, based on the earlier FEV Curling-13 strain used by the Enclave in 2241, only now it was designed to be placed into a water purification system, it is received after dialogue with President John Henry Eden.
According to President Eden it will wipe out super mutants, ghouls, and all other "unclean" citizens, namely anyone who had been born in the Wastelands and exposed to their radiation and other mutational effects from birth. The only people who would survive the infection are dangerous radiation level free humans and humans who were born in the Vaults. It should be noted that at this point it has been established in the plot that the Lone Wanderer was not in fact born in a vault, but there is no indication of whether President Eden is aware of this.
In the add-on Broken Steel, if the player had chosen to contaminate the Purifier with the Modified FEV, the medical facility of every city in the Capital Wasteland (except Tenpenny Tower) is filled with new patients dying from FEV infection. The new patients hold their stomachs and repeat dialogue along the lines of, "it burns on the inside." When the player speaks with the doctor at any given town, they state that the condition is not contagious. In Rivet City, Doctor Preston comments that they are burning the corpses just to be safe. In Megaton, Doc Church makes a comment along the lines of, "it'd be damned ironic if we were so used to the crap we had been drinking that actual pure water was killing us." If the Lone Wanderer drinks four bottles of Aqua Pura with the modified FEV, they will die.
- You will not be able to leave the room where President Eden is located unless you pick up the virus.
- This item uses the world model of an empty Nuka-Cola bottle, likely because, as a quest item, it can never be dropped by the player.
- It can be given to Elder Lyons for good Karma if you talk to him after you've escaped Raven Rock. With Broken Steel, the Modified FEV will be removed from the player's inventory if they activated Project Purity but did not give the Modified FEV virus to Elder Lyons beforehand.
- Though the player is told it will only infect mutants, ghouls and the "unclean," there are very few "pure" humans without any mutations or chromosomal damage left in the Wasteland, due to long-term exposure to radiation over many generations. This includes the Lone Wanderer.